The separation-of-power issue in the case is obvious: When Congress enacts a law in direct opposition to the President's practice (which, in turn, is driven by the President's foreign policy), who wins? But the case may turn on a more basic question: Separation of powers about what?

The plaintiffs in the case argued that the case is merely about passports, and not foreign policy. They said that the case can be decided easily under Justice Jackson's three-part framework in the steel-seizure case, Youngstown Sheet & Tube Co. v. Sawyer: When Congress acts pursuant to its own authority, as here, the President's power is at its lowest ebb. The plaintiffs translated this as a kind of congressional veto over executive power--that when Congress and the President clash, Congress wins.

But more: The plaintiffs seemed to argue that even if the passport power implicates foreign affairs, Congress has a "shared" power with the President over foreign affairs. This leaves little exclusive power to the President over foreign affairs--an approach to executive authority over foreign affairs that Justice Kennedy called "crabbed." Others on the bench also signalled difficulties with this position; for example, Justice Sotomayor showed how it would "hobbl[] the President with respect to situations that occur frequently [in foreign affairs]," like changes in governments and changes in sovereignty. Justice Scalia put perhaps the finest point on all this:

Mr. Lewin, you're--it seems to me you are not arguing for a co-equal congressional power, you are arguing for a superior congressional power. You are saying whatever Congress says, the President has to comply with. Now, that's quite different from saying that they both have authority in the field.

Transcript, 10-11.

There was an even more basic problem with the plaintiff's approach, though. That is: What exactly is Congress's authority over passports, and where does it come from? The plaintiffs didn't have a great answer for that question (from Justice Kagan).

But even with these problems with the plaintiffs' approach--its all-or-nothing nature, its rigidity, and its lack of textual support--the argument was by no means one-sided. The government similarly dug in its heels on its position on executive authority in foreign affairs: The President has exclusive authority, leaving no room for Congress. But it's not obvious that such expansive authority here derives from the text--the government only has the reception clause (its power to receive ambassadors) and its historical gloss on that power. And the government's theory didn't adequately address how it squares with Congress's power of the purse, its advice-and-consent power over appointments, and its oversight authority--whether those quite clear congressional authorities could in effect override the President's execution of the foreign affairs power. (The government said that a congressional act defunding a foreign affairs policy might raise constitutional problems, but it didn't say why. This is exactly what the government faced with congressional defunding of transportation of Guantanamo detainees to the mainland for criminal trials in Article III courts. The Obama administration balked, but only a little, and basically acquiesced in that act of congressional control over a foreign affairs matter.)

Moreover, the Court was quick to recognize that the government's aggressive position on the political question doctrine--that the issue here is textually delegated to the President alone, and therefore the courts shouldn't intervene--answers the underlying merits question. That is, to decide that the Constitution gives the power to the President for the purpose of the political question doctrine is also to decide that the Constitution gives the power to the President for the purpose of actualizing the power. This didn't seem to sit well.

Some on the bench floated intermediate positions. On the merits, both Congress and the President probably have some power over place-of-birth designation on passports, but that that power might be very different. The President may have some power by way of recognition, derived from the reception clause; but Congress, too, may have power by way of appropriations, appointments, and oversight. On the political question doctrine, this case could well be a "political question" (or otherwise nonjusticiable) for that very reason--that both political branches have some power, but that their powers are different, and that the courts should let them work it out (as they do in so many issues). These intermediate positions offer a more moderate and appealing view of shared power than the extreme views of either side in the case, and they keep the Court well away from delving into the underlying foreign policy itself--something that many on the Court seemed concerned about.